Method and system for providing fuel gas to a topside facility

ABSTRACT

A method for providing fuel gas to a top-side processing system of a sub-sea well stream is provided. The method comprises separating the sub-sea well stream in a sub-sea arranged three phase separator obtaining a crude oil stream, and a crude natural gas stream, separating acid gasses and/or water from the crude natural gas, pressurizing the gas stream sub-sea, adding at least part of the pressurized gas to the crude oil stream subsea, transporting the added gas together with the oil to the topside, separating the gas from the oil in a first stabilization stage and providing the separated gas as fuel gas to a fuel gas system.

The present invention relates to an offshore processing method, especially the present invention relates to an offshore processing method for providing fuel for an offshore processing system which can be combined with a method for stabilization of oil for tanker transport.

BACKGROUND

The exploitation of subsea wells has gone through considerable developments since the start of the offshore era. The location of the wells has changed both with respect to sea depth as well as distance to shore. This has resulted in new systems for handling the well streams. One such system includes a subsea installation near the well head combined with a topside platform, which can be a floating processing platform. Transportation of the well products can take place either through subsea pipelines or via tanker transport. Due to the limited space on a floating platform generally the amount of process equipment need to be adjusted to the available space. Transport of crude oil on tankers requires that the crude oil has been stabilized to an extent that gas which can be released from the oil during transport is limited. The release of gas during transport is a result of changes in the conditions for instance due to changes in the temperatures of the surroundings, but also a result of settlement over time during transport.

Stabilization of crude oil for tanker transport comprises controlled degasification of the crude oil.

The offshore top side processing equipment requires power for driving mechanical equipment and for providing heat.

The power can be provided through electrical cables connected to onshore power plants or be generated offshore through combustion of fuel or a combination thereof.

PRIOR ART

Stabilisation of crude oil through degasification is well known in the art, and WO 03/033872 is mentioned as an example.

WO 03/033872 relates to an installation arranged on the sea bed for the separation of fluids, comprising at least one separator that is connected to one or more wells, each via an associated well head or similar, and a pipeline. The components separated, oil, gas, water or combinations of these substances, are fed fully or partially from the installation to a platform, vessel etc. on the surface via collecting pipelines onto shore, or are re-injected into the formation beneath the sea bed. Each separator consist of a long pipe (pipe separator) that may form a major or minor part of the transport pipeline from the well and has a diameter that is mainly equal to or slightly larger that the diameter of the transport pipeline.

OBJECTIVES OF THE INVENTION

An aim of the present invention is to provide a method for providing fuel to a topside platform from a subsea well which utilizes equipment mainly provided for other purposes.

Another aim is to provide a method for stabilisation of oil for tanker transport.

The present invention provides a method for providing fuel gas to a top-side processing system of a sub-sea well stream, where the method comprises separating the sub-sea well stream in a sub-sea arranged three phase separator obtaining a crude oil stream, and a crude natural gas stream, separating acid gasses and/or water from the crude natural gas, pressurizing the gas stream sub-sea, adding at least part of the pressurized gas to the crude oil stream subsea, transporting the added gas together with the oil to the topside, separating the gas from the oil in a first stabilization stage and providing the separated gas as fuel gas to a fuel gas system.

In one aspect of method according to the present invention the method further comprises removing water from the crude oil stream subsea to obtain dry crude oil, transporting the dry crude oil to the topside processing system. Removing water from the crude oil stream subsea may in another aspect comprise passing the crude oil trough a subsea arranged electrostatic coalescer.

In a further aspect of the method according to the present invention the crude oil stream is stabilized for tanker transport in the top-side processing system through removal of dissolved gas.

In yet another aspect of the method according to the present invention the first stabilization stage operates at a gas pressure equal to a pressure required for the topside fuel gas system.

In an additional aspect of the present invention the crude oil is stabilized in the first stage and in a second stage with interstage heating.

In a further aspect of the method according to present invention the step of separating acid gasses and/or water from the crude natural gas is performed subsea.

Further the present invention provides a subside well, topside fuel gas providing system comprising a sub-sea arranged three phase separator with a well stream inlet and at least a crude natural gas outlet and a crude oil outlet, a riser with a subsea riser inlet in fluid communication with the crude oil outlet and in fluid communication with the crude natural gas outlet and a topside riser outlet in fluid communication with a topside arranged stabilization unit comprising a stabilized crude oil outlet and a fuel gas outlet.

In one aspect of the system according to the present invention, the system further comprises a subsea gas treatment system down stream the three phase separator with an inlet in fluid communication with the crude natural gas outlet and with a treated gas outlet in fluid communication with the riser.

In a further aspect the system comprises a sub-sea arranged water removal unit arranged downstream the three phase separator with a crude oil inlet in fluid communication with the crude oil outlet, a dry crude oil outlet in fluid communication with the riser and a water outlet. In one aspect of the system the water removal unit comprises an electrostatic coalescer.

In yet another aspect the system according to the present invention the fuel gas outlet is in fluid communication with a fuel gas inlet to a topside arranged power generating unit.

In an additional aspect of the system according to the present invention the system further comprises a subsea natural gas compressor with an inlet in fluid communication with the natural gas outlet and a pressurized gas outlet in communication with the riser.

The term “topside” as used here refers to a position in proximity of the sea level. For floating topside installations part of the equipment may be installed above or below the sea level but with in or on the floating vessel or platform. For platforms with one or more legs connected to the seabed the term “topside” should be interpreted to refer to any position on the platform above sea level.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed in further detail with reference to the enclosed figures where:

FIG. 1 schematically illustrates a first embodiment of the present invention.

FIG. 2 schematically illustrates a second embodiment of the present invention.

FIG. 3 schematically illustrates a third embodiment of the present invention.

FIG. 4 illustrates the process scheme of a fourth embodiment of the present invention.

PRINCIPAL DESCRIPTION OF THE INVENTION

The figures illustrated different embodiments of the present invention. The same reference numbers are used to refer to equally elements within the different embodiments. It should be understood that features of the different embodiments may be combined to provide additional embodiments of the present invention.

FIG. 1 illustrates a first embodiment of the present invention. A well stream 1 is fed to a subsea three-phase separator 2, to obtain a water stream 19, a crude oil stream 13 and a gas stream 3. The water stream 19 is transferred to a produced water treatment system 4. The gas stream 3 is past through a heat exchanger 8 to obtain a cooled gas 5. The cooling results in condensation of higher hydrocarbons which are separated in separator 10. The liquid stream 7 is via pump 12 and pipe 9 mixed with the main oil stream from the separator 2. The gas 11 leaving over the top of the separator 10 is subject to a treatment system 22 conditioning the gas for pipeline transport. The conditioned gas 51 is pressurized in subsea compressor 52 to obtain pressurised gas 53.

The oil stream 81 comprising the main crude oil stream 13 and the condensed oil 9 is transported via a pump 80 and a pipeline 83. A part 57 of the pressurized gas is added to the oil before the oil is transferred to top side via line 85. The rest of the gas is transferred via heat exchanger 54 and pipeline 55 to shore.

The mixture of gas and oil is transported to a topside installation 90 in riser 85. Within the oil treatment system 6′ the oil is dehydrated and gas initially present as well as the added gas is removed in a first stabilization step 84. Stream 87 is stabilized oil, depending on the restrictions for tanker transport additional oil treatment may if necessary be performed on the topside installation. Stream 89 comprises the added gas stream 57 as well as any additional gas released from the crude oil. This stream 89 is fed as fuel gas to a topside gas fuel power providing system 86.

FIG. 2 illustrates an alternative embodiment of the present invention, wherein the at least partly stabilized crude oil 87 is returned from the topside installation to subside, and transported to shore together with the compressed gas 55 as stream 65 via a subsea pipeline. In this embodiment the crude oil is transferred to the topside installation to be dried. For pipeline transport together with the crude gas both the gas and the oil need to be dried to avoid hydrate formation.

FIG. 3 illustrates a further embodiment of the present invention. Here also the initial oil treatment system 6 is arranged subsea. Here the crude oil is dehydration for instance by use of electrostatic coalescers. Separated water is past as stream 15 to the produced water treatment system 4. The dehydrated oil 81 is via pump 80 transferred to the topside installation 90 together with treated gas 57. The stabilization system 84 comprises one or more stabilization steps where gas is flashed of from the crude oil. The first step is preferably performed at a pressure equal to the pressure required by the fuel gas system 86 so that it is not required to compressed the fuel gas 89 before feeding it to the fuel gas system 86. The fuel gas is a combination of the added gas 57 and the gas present in the crude oil prior to stabilization. Additional stabilization steps may be included within unit 84 to allow for removal of additional gas to secure quality applicable for tanker transport. These additional steps will be performed at a lower pressure than the pressure of the fuel gas. If significant amounts of gas are released at a lower pressure, then the treatment system may also comprise a compressor to increase the pressure of the removed gas to the pressure required by the fuel system. Alternatively, if there are two fuel gas systems topside; 1) for feeding the generator turbine (high pressure) and 2) one for feeding a direct fired heater to provide heating medium, then the gas released at lower pressure is fed to the fuel system for the direct fired heater. Alternatively if the pressure of the removed gas obtained in the first step is larger than the pressure needed for the fuel system. The two gas streams, from the first step with to high pressure and from the additional step with to low pressure may be combined before they are fed to the fuel gas system with the correct pressure.

FIG. 4 illustrated an embodiment of the present invention in further detail. The well fluid 1 enters a phase separator 2, where the gas stream 3 is separated from the liquid. In the illustrated embodiment a water stream 19 is passed to a produced water treatment system 4, and the oil stream 13 is past to an oil treatment system 6. The gas 3 is initially cooled by cooler 8 to obtain cooled gas 5. The cooling results in condensation of higher hydrocarbons which are separated in separator 10. The liquid stream 7 is via pump 12 and pipe 9 mixed with the main oil stream from the separator 2. The gas 11 leaving over the top of the separator 10 is subject to a gas treatment system. In FIGS. 1-3 this was referred to as system 22 whereas here one embodiment of this system is disclosed in detail. However it should be noted that other subsea systems for gas conditioning are equally applicable. The crude gas stream 11 comprising a compound to be removed such as CO₂ and/or other acid gasses is fed to a contactor system 14, 16 arranged subsea. In the illustrated embodiment the contactor system is a 2 stage process with an initial direct contactor 14 and a traditional contactor column 16. However the present invention is not limited to this embodiment but any contactor system applicable for subsea arrangement may be employed. In the direct contactor the gas stream is brought in contact with a treatment solution stream fed trough pipe 23. The obtained gas treatment solution mixture proceeds as stream 17 into the contactor column 16. Lean treatment solution is provided to the column from pipe 21. In the illustrated embodiment the treatment solution for the direct contactor is obtained from the column 16 at a level above the gas inlet, however the present invention is not limited to this solution as lean treatment solution could also be fed to the direct contactor as well as the column. The treatment solution comprises one or more species that at least with some selectivity absorb the compound to be removed from the crude gas stream. A species and solvent/diluent applicable for forming an effective treatment solution can be selected by the user depending on the compound to be removed and the prevailing conditions within the system. During contact with the treatment solution the compound to be removed is absorbed in the solution. The crude gas accordingly at least partly depleted from the compound to be removed leaves over the top of the column trough pipeline 31. The main crude gas stream accordingly stays subsea during the treatment process. The rich treatment solution leaves the contactor 16 trough the bottom outlet pipeline 25.

In the illustrated embodiment a pump 18 is provided to force the rich treatment solution 25 to proceed up through the riser or pipeline 27. The is only an illustration of one possible way of securing transport of the rich treatment solution up trough the pipeline 27 other methods of providing the needed pressure and flow can equally be employed. On a topside facility 90 a regeneration system 20 is installed. The system receives the rich solution from pipeline 27, desorbes and separates the absorbed compound there from and obtains a depleted treatment solution which is send back to the subsea contactor system trough pipeline/riser 21. The desorbed compound leaves the regeneration unit 20 as stream 29. If the compound is CO₂ the stream 29 may be treated and compressed in unit 60 and from there transported via pipeline 61 to a subsea injection well (not shown).

The obtained treated gas 31 enters a second system for removal of a second compound, such as water. The system comprises a direct contactor 34, where the gas is brought in contact with a treatment solution stream 43. The obtained mixture is fed to a contactor column 36. The rich treatment solution leaves via the bottom as stream 45, via pump 38 and is transported via pipeline 47 to a top side installation 90 and a regeneration unit 40. Here the compound absorbed in the rich treatment solution is released resulting in a compound stream 49 and a lean treatment solution 41 being returned to the subsea contactor 36. If the compound is water the stream 49 is steam that can be released to the atmosphere.

The treated gas stream 51 leaving over the top of 36 can be compressed in compressor 52 and the temperature of the compressed gas 53 controlled by heat exchanger 54. Hereby providing a treated gas stream 55 adjusted for subsea pipeline transport. 

1. A method for providing fuel gas to a topside processing system for a subsea well stream, the method comprising: separating the subsea well stream in a subsea arranged three phase separator to thereby obtain a crude oil stream and a crude natural gas stream; separating at least one of acid gasses and water from the crude natural gas stream; pressurizing the crude natural gas stream subsea; adding at least part of the pressurized gas to the crude oil stream subsea; transporting the added pressurized gas together with the crude oil to the topside processing system; separating the pressurized gas from the crude oil in a first stabilization stage; and providing the separated gas as fuel gas to a fuel gas system.
 2. The method according to claim 1, further comprises removing water from the crude oil stream subsea to obtain dry crude oil, and transporting the dry crude oil to the topside processing system.
 3. The method according to claim 2, wherein the step of removing water from the crude oil stream subsea comprises passing the crude oil through a subsea arranged electrostatic coalescer.
 4. The method according to claim 1, 2 or 3, further comprising stabilizing the crude oil stream for tanker transport in the topside, processing system through removal of dissolved gas from the crude oil stream.
 5. The method according to claim 4, wherein the stabilizing step is performed at a gas pressure equal to a pressure required for the topside fuel gas system.
 6. The method according to claim 4, wherein the crude oil is stabilized in first and second stages and is heated with interstage heating.
 7. The method according to claim 1, wherein the step of separating acid gasses or water from the crude natural gas is performed subsea.
 8. A subsea well topside fuel gas-providing system which comprises: a subsea arranged three phase separator which includes a well stream inlet, a crude natural gas outlet and a crude oil outlet; a riser which includes a subsea riser inlet in fluid communication with the crude oil outlet and the crude natural gas outlet; the riser further comprising a topside riser outlet in fluid communication with a topside arranged stabilization unit which includes a stabilized crude oil outlet and a fuel gas outlet.
 9. The system according to claim 8, further comprising a subsea gas treatment system located downstream of the three phase separator, the subsea gas treatment system comprising an inlet in fluid communication with the crude natural gas outlet and a treated gas outlet in fluid communication with the riser.
 10. The system according to claim 8 or 9, further comprising a subsea arranged water removal unit located downstream of the three phase separator, the water re oval unit comprising a crude oil inlet in fluid communication with the crude oil outlet, a dry crude oil outlet in fluid communication with the riser, and a water outlet.
 11. The system according to claim 10, wherein the water removal unit comprises an electrostatic coalescer.
 12. The system according to claim 8, herein the fuel gas outlet is in fluid communication with a fuel gas net inlet of a topside arranged power generating unit.
 13. The system according to claim 8, further comprising a subsea natural gas compressor with an inlet in fluid communication with the natural gas outlet and a pressurized gas outlet in communication with the riser. 